In recent years, environmental disruption has been at issue, and energy saving and CO2 reduction technologies have been attracting attention. Accordingly, further improvement in the combustion efficiency of internal combustion engines has been desired, and development of materials having improved high-temperature properties has been demanded.
For such a demand, (1) Ni-based superalloys and (2) Ni-based dual multi-phase intermetallic compound alloys are being developed as the materials having improved high-temperature properties.
The Ni-based superalloys mentioned as (1) have a γ phase (Ni solid solution phase), which is a parent phase, and a γ′ phase, which has dispersed and precipitated in the parent phase. The γ′ phase is an intermetallic compound having a basic composition of Ni3Al (L12 phase) and accounts for approximately 60 to 70 vol % of the constituent phases.
The alloys have been developed into ordinary cast alloys, into directionally solidified alloys, and into single crystal alloys. The single crystal alloys have been developed into first generation alloys, into second generation alloys containing approximately 3% by weight of Re, into third generation alloys containing 5 to 6% by weight of Re, into fourth generation alloys containing 2 to 3% by weight of a noble metal such as Ru, and into fifth generation alloys containing 5 to 6% by weight of a noble metal.
As an Ni-based superalloy for a directionally solidified material, for example, an Ni-based directionally solidified superalloy containing C, B, Hf, Co, Ta, Cr, W, Al and Re, and a balance made up of Ni and inevitable impurities is known (see Patent Document 1, for example).
This alloy can contain Ti, Nb, V and Zr as an optional component and is improved in strength in the solidification direction and in strength at grain boundaries by adjusting the amounts of elements composing the γ phase as a parent phase and the γ′ phase as a precipitate phase, and by adjusting the amounts of elements for strengthening the grain boundaries.
Meanwhile, as an Ni-based single crystal superalloy having both high-temperature strength and oxidation resistance at high temperature in practical use in a balanced manner, an Ni-based single crystal superalloy containing Al, Ta, W, Re, Cr and Ru as main elements is known (see Patent Document 2, for example).
For this alloy, the high-temperature strength (creep strength) is improved by determining the composition ratio among the elements in the most suitable range, and thereby controlling the lattice constant of the parent phase (γ phase) and the lattice constant of the precipitate phase (γ′ phase) to the most appropriate values.
These Ni-based superalloys are developed from the viewpoint of high-temperature strength and casting, because they are mainly applied to turbine blades in jet engines and the like, and therefore elements preferable in this view are added to the composition. As described above, the Ni-based superalloy includes a γ phase as a parent phase and a γ′ phase as a precipitate phase. In this regard, it is explained that Re is dissolved in the γ phase (solid solution phase) and improves the creep strength (see Patent Documents 1 and 2, for example). It is also explained that Ta as well as W is dissolved in the γ phase and a part thereof is dissolved in the γ′ phase, and improves the creep strength (see Patent Document 2, for example). It is further explained that V reduces the high-temperature strength and therefore the amount thereof is preferably 1% by weight or less (see Patent Documents 1 and 2, for example).
However, the γ phase, which is a metallic phase accounts for approximately 30 to 40 vol % or more of the constituent phases of the Ni-based superalloy, and therefore the drawback is that the melting point and the high-temperature creep strength of the superalloy are low. In addition, while the development from the viewpoint of high-temperature strength has been advanced, development from the viewpoint of hardness has not been advanced.
Meanwhile, the Ni-based dual multi-phase intermetallic compound alloys mentioned as (2) above are expected to be developed as alloys that can solve such problems. The Ni-based dual multi-phase intermetallic compound alloys are multi-phase alloys obtained by combining Ni3X-type intermetallic compounds belonging to geometrically closed packed crystal structures with crystallographic coherency. For example, an Ni-based dual multi-phase intermetallic compound alloy is composed of an Ni3Al intermetallic compound phase called as the γ′ phase and an Ni3V intermetallic compound phase.
FIG. 17 is a drawing for illustrating a microstructure of the Ni-based dual multi-phase intermetallic compound alloy. In FIG. 17, (1) shows an exemplary SEM photograph for illustrating a microstructure of the Ni-based dual multi-phase intermetallic compound alloy (Ni75Al8V14.5Nb2.5), and (2) shows schematic views of crystal structures (Ni3Al, Ni3V) composing the microstructure of the Ni-based dual multi-phase intermetallic compound alloy.
As shown in FIG. 17, the Ni-based dual multi-phase intermetallic compound alloy is composed of a microstructure formed with crystallographic coherency and a nanostructure formed in channels of the microstructure (see FIG. 17 (1)). The former microstructure is composed of a primary precipitate L12 phase (Ni3Al shown in FIG. 17 (2)), and the nanostructure is composed of an eutectoid structure consisting of an L12 phase and a D022 phase (Ni3Al and Ni3V shown in FIG. 17 (2)).
The Ni-based dual multi-phase intermetallic compound alloy is composed of an upper multi-phase microstructure formed of the primary precipitate L12 phase precipitating in an A1 phase (Ni solid solution phase) through a heat treatment at a temperature higher than the eutectoid point; and a lower multi-phase microstructure formed of two phases including the L12 phase and the D022 phase generated by eutectoid transformation of the A1 phase through a subsequent heat treatment at a temperature lower than the eutectoid point.
As described above, the Ni-based dual multi-phase intermetallic compound alloy is formed from multi-phased Ni3X-type intermetallic compounds having excellent properties. Accordingly, the Ni-based dual multi-phase intermetallic compound alloy has superior properties to alloys composed of a single intermetallic compound phase and is expected as an alloy that can allow wide-ranging microstructural control (see Patent Document 3). The Ni-based dual multi-phase intermetallic compound alloys are being developed from the viewpoint of hardness as well as of high-temperature strength, for example.
As a specific example of an Ni-based dual multi-phase intermetallic compound alloy showing excellent hardness not only at room temperature but also at high temperature, an alloy containing Ni as a main component, and Al, V, Ta and/or W, Nb, Co, Cr and B is known (Nb, Co and Cr are optional components) (see Patent Document 4).
In addition, as an Ni-based dual multi-phase intermetallic compound alloy increased in surface hardness, an Ni-based dual multi-phase intermetallic compound alloy is known in which a base material is an alloy containing Ni as a main component, and Al, V, Nb, Ti, Co, Cr and B (Nb, Ti, Co and Cr are optional components), and the base material is surface-treated by at least one of nitridation and carburization (see Patent Document 5).